vfs: Add a single-or-reconfig keying to vfs_get_super()

Add an additional keying mode to vfs_get_super() to indicate that only a
single superblock should exist in the system, and that, if it does, further
mounts should invoke reconfiguration upon it.

This allows mount_single() to be replaced.

[Fix by Eric Biggers folded in]

Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/super.c

···
 {
 	int (*test)(struct super_block *, struct fs_context *);
 	struct super_block *sb;
+	int err;
 
 	switch (keying) {
 	case vfs_get_single_super:
+	case vfs_get_single_reconf_super:
 		test = test_single_super;
 		break;
 	case vfs_get_keyed_super:
···
 		return PTR_ERR(sb);
 
 	if (!sb->s_root) {
-		int err = fill_super(sb, fc);
-		if (err) {
-			deactivate_locked_super(sb);
-			return err;
-		}
+		err = fill_super(sb, fc);
+		if (err)
+			goto error;
 
 		sb->s_flags |= SB_ACTIVE;
+		fc->root = dget(sb->s_root);
+	} else {
+		fc->root = dget(sb->s_root);
+		if (keying == vfs_get_single_reconf_super) {
+			err = reconfigure_super(fc);
+			if (err < 0) {
+				dput(fc->root);
+				fc->root = NULL;
+				goto error;
+			}
+		}
 	}
 
-	BUG_ON(fc->root);
-	fc->root = dget(sb->s_root);
 	return 0;
+
+error:
+	deactivate_locked_super(sb);
+	return err;
 }
 EXPORT_SYMBOL(vfs_get_super);
 
···
 	return vfs_get_super(fc, vfs_get_single_super, fill_super);
 }
 EXPORT_SYMBOL(get_tree_single);
+
+int get_tree_single_reconf(struct fs_context *fc,
+		  int (*fill_super)(struct super_block *sb,
+				    struct fs_context *fc))
+{
+	return vfs_get_super(fc, vfs_get_single_reconf_super, fill_super);
+}
+EXPORT_SYMBOL(get_tree_single_reconf);
 
 int get_tree_keyed(struct fs_context *fc,
 		  int (*fill_super)(struct super_block *sb,

include/linux/fs_context.h

···
  */
 enum vfs_get_super_keying {
 	vfs_get_single_super,	/* Only one such superblock may exist */
+	vfs_get_single_reconf_super, /* As above, but reconfigure if it exists */
 	vfs_get_keyed_super,	/* Superblocks with different s_fs_info keys may exist */
 	vfs_get_independent_super, /* Multiple independent superblocks may exist */
 };
···
 			 int (*fill_super)(struct super_block *sb,
 					   struct fs_context *fc));
 extern int get_tree_single(struct fs_context *fc,
+			 int (*fill_super)(struct super_block *sb,
+					   struct fs_context *fc));
+extern int get_tree_single_reconf(struct fs_context *fc,
 			 int (*fill_super)(struct super_block *sb,
 					   struct fs_context *fc));
 extern int get_tree_keyed(struct fs_context *fc,